Gas sensor

ABSTRACT

If a part of the surface of a solid electrolyte (1) is covered with a layer (2) of a metal salt capable of forming a dissociative equilibrium with a gas component to be measured and the remaining surface of the solid electrolyte (1) is substantially completely sealed and coated with a gas-intercepting layer (3), there can be provided a gas sensor having a simple structure, the size of which can be drastically reduced, and the operation characteristics of this gas sensor are very stable.

TECHNICAL FIELD

The present invention relates to a gas sensor. More particularly, thepresent invention relates to a structure of a novel solid electrolytetype gas sensor for measuring the concentrations of various gascomponents such as CO₂, Cl₂, SO_(x), and NO_(x).

BACKGROUND ART

A solid electrolyte is a substance in which an electric current iscaused to flow by migration of ions in the solid. If a solid electrolytecell, in which a gas component to be detected, contained in the gas tobe tested, is used as a reactant, is fabricated by using such a solidelectrolyte, the concentration of the intended gas component can beknown from the amplitude of the electromotive force of the cell or anelectric current that can be taken out from the cell. This gas sensor iscalled "a solid electrolyte type gas sensor".

Various solid electrolyte type gas sensors differing in the structurehave been proposed (see Mizusaki, Yamauchi and, Fueki, "Ideas of SolidElectrolyte Sensors", Electrical Chemistry and Industrial PhysicalChemistry, Vol. 50, No. 1, pages 7-12, 1982, Yamauchi and Fueki, "SolidElectrolyte Gas Sensors", ibid, pages 46-53, and literature citedtherein). In most conventional solid electrolyte type gas sensors, areference electrode to be contacted with a standard gas should bedisposed, and therefore, the structure is complicated and the size isinevitably large. On the other hand, solid electrolyte type gas sensorshaving a structure in which a standard gas is not necessary aredefective in that a satisfactory stability cannot be obtained.Accordingly, only a stabilized zirconia O₂ sensor is practically used atthe present.

DISCLOSURE OF THE INVENTION

Under this background we made research and, as a result, found that if apart of the surface of a solid electrolyte is covered with a layer of ametal salt capable of forming a dissociative equilibrium with a gascomponent to be measured and the remaining part of the surface of thesolid electrolyte is substantially completely sealed and covered with agas-intercepting layer so as to prevent contact with the ambientatmosphere as much as possible, there can be obtained a solidelectrolyte type gas sensor in which the structure is simplified, thesize is drastically reduced, and the operation characteristics aresufficiently stable. We have completed the present invention based onthis finding.

More specifically, in accordance with the present invention, there isprovided a gas sensor comprising a solid electrolyte, a measurementelectrode attached to a part of the surface of the solid electrolyte, alayer of a metal salt capable of forming a dissociative equilibrium witha gas component to be measured, which covers the surface of themeasurement electrode and the measurement electrode-surrounding regionof the surface of the solid electrolyte, a reference electrode attachedto a part of the surface, not covered with the metal salt layer, of thesolid electrolyte, a gas-intercepting layer covering the surface of thereference electrode and substantially all of the remaining surface, notcovered with the metal salt layer, of the solid electrolyte, and leadlines for taking out potentials from the measurement electrode and thereference electrode.

This gas sensor is prominently characterized in that a standard gas isnot necessary and the gas sensor has a sealed structure in which onlythe metal salt layer falls in contact with the measurement atmospheresuch as the gas to be measured, apart from the gas-intercepting layer,and the solid electrolyte is completely isolated from the ambientatmosphere. The gas sensor of the present invention is decisivelydistinguishable over the conventional techniques in the above-mentionedcharacteristic feature.

The operation mechanism of the gas sensor of the present inventionhaving the above-mentioned structure has not been completely elucidated,but in view of the fact that, as illustrated in the examples givenhereinafter, the gas sensor of the present invention has a sufficientoperation stability and the generated electromotive force is in accordwith Nernst's equation, it is presumed that a very simple cell systemdepending only on the dissociative equilibrium of the metal salt mayprobably be realized stably. Namely, in the conventional gas sensorstructures, isolation of a solid electrolyte, which is ordinarily ahighly reactive substance, from the ambient gas is not sufficientlytaken into account, but the solid electrolyte is positively exposed tothe ambient gas, and therefore, it is considered that, due to difficultto control disturbance factors, attainment of stability andreproducibility is difficult.

According to the structure of the gas sensor of the present invention,the concentration of a gas component to be measured can be substantiallymeasured just based on the dissociative equilibrium reaction of themetal salt. Therefore, if the kind of the metal salt or solidelectrolyte is appropriately selected, the application range of the gassensor can be broadened with ease.

A metal salt capable of forming a dissociative equilibrium with a gascomponent to be measured, such as CO₂, Cl₂, SO_(x), or NO_(x), is usedfor the metal salt layer of the gas sensor. For example, there can bementioned Li₂ CO₃, Na₂ CO₃, BaCO₃, SrCO₃, CaCO₃, MgCO₃, PbCO₃, FeCO₃,ZnCO₃ ; NaCl, AgCl, CaCl₂, PbCl₂, CdCl₂, CuCl₂, CoCl₂, MnCl₂, NiCl₂,FeCl₂, ZnCl₂, BiCl₂, MgCl₂, VCl₂ ; Na₂ CO₄, BaSO₄, SrSO₄, CaSO₄, Ag₂ SO₄, CdSO₄, NiSO₄, ZnSO₄, CoSO₄ ; NaNO₃, Ba(NO₃)₂, Ca(NO₃)₂ , and AgNO₃.The metal salt layer may be formed by coating the metal salt directly onthe surface of the solid electrolyte or for example by mixing the metalsalt with a ceramic powder and sintering the mixture on the surface ofthe solid electrolyte. Furthermore, the metal salt may be formed into afilm having a thickness of about 1 μm by sputtering.

As pointed out hereinbefore, selection of an appropriate metal salt canbe presupposed to some extent based on Nernst's equation according tothe application conditions of the sensor, but, generally, a metal salthaving a higher dissociative equilibrium partial pressure is preferred.For example, Li₂ CO₃ and Na₂ CO₃ have a high dissociative equilibriumpartial pressure and hence, they are preferred.

A conductor for the metal ion of the metal salt used may be used as thesolid electrolyte of the gas sensor. For example, as the solidelectrolyte having a lithium ion as the electroconductive carrier, therecan be mentioned Li/β-alumina, Li₁₄ (CeO₄)₄, etc., and as the solidelectrolyte having a sodium ion as the electroconductive carrier, therecan be mentioned Na/β"-alumina, Na/β-alumina, Na₂ Zr₂ PSi₂ O₁₂, Na₃ Zr₂Si₂ PO₁₂ (NASICON), Na/β-Ga₂ O₃, Na/Fe₂ O₃ etc. Furthermore, as thesolid electrolyte having a potassium ion as the electroconductivecarrier, there can be mentioned K/β-alumina and K₁.6 Al₀.8 Ti₇.2 O₁₆,and as the solid electrolyte having a calcium ion as theelectroconductive carrier, there can be mentioned CaS etc. These solidelectrolytes are described in detail in the above-mentioned literatureand the references cited therein For example, Li/β-alumina is expressedas Li₂ O.11Al₂ O₃, LiO₂ is intruded in layers of spinel blocks composedof alumina, and Li⁺ migrates easily in the layers.

There is a relation between the shape of the solid electrolyte and theresponse time of the gas sensor, so the response time can be shortenedby reducing the thickness of the film of the solid electrolyte.

The above-mentioned solid electrolytes can be easily prepared and arecommercially available.

The gas-intercepting layer of the gas sensor of the present inventionshould be substantially gas-impermeable and electrically insulating.Furthermore, the gas-insulating layer should have a heat resistanceaccording to the temperature at which the gas sensor is used. Ceramics,glass, plastics etc. may be used.

For the electrode of the gas sensor, there may be ordinarily used gold,silver, platinum, graphite, LaCoO₃ (inclusive of itsnon-stoichiometrically sintered product), and a ferrite electrodematerial such as (K,Na)₂ O.67(Fe₁.97 T₀.05 O₃). When the electrodematerial is selected, the following factors are taken intoconsideration. Namely, the diffusion of chemical seeds should be large(the ion conductivity should be large), the electroconductivity shouldbe high, the chemical stability should be high such that the electrodematerial does not react with the electrolyte, the crystallographicstability should be high such that phase transition is not caused at thetemperature at which the gas sensor is used, the vapor pressure shouldbe so low that volatilization is not caused, the metallographicstability should be so high that the electrode material does not reactwith a conductor line, the mechanical strength should be sufficient, thethermal expansion coefficient of the electrode material should be inaccord with that of the electrolyte so that the interface peeling is notcaused, and the electrode material should be economically advantageous.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional diagram illustrating an example of the structureof the gas sensor of the present invention;

FIG. 2 is a graph showing the CO₂ partial pressure dependency of thegenerated electromotive force of a gas

sensor having a structure of Ag/β-Al₂ O₃ /Na₂ CO₃,Ag(CO₂) according toan example of the present invention; and

FIG. 3 is a graph showing the CO₂ partial pressure dependency of thegenerated electromotive force of a gas sensor having a structure ofAg/β-Al₂ O₃ /Li₂ CO₃,Ag(CO₂) according to another example of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a sectional diagram illustrating an example of the structureof the gas sensor of the present invention. A solid electrolyte 1 has,for example, a columnar (disc-like), sheet-like, or filmy shape,typically a columnar (disc-like) shape. A measurement electrode 4 and areference electrode 5 are attached to two main faces of the solidelectrode 1, respectively, and lead lines 6 are connected to theelectrodes 4 and 5. A metal salt layer 2 covers a part of the surface ofthe solid electrolyte 1 inclusive of the surface of the measurementelectrode 4. The remaining part, not covered with the metal salt layer2, of the surface of the solid electrolyte 1 is substantially completelysealed and covered with a gas-intercepting layer 3. Accordingly, in thisgas sensor, as pointed out hereinbefore, it is only the metal salt layer2 that falls in contact with the ambient gas atmosphere to be measured,and the solid electrolyte 1 is completely isolated from the ambientatmosphere.

EXAMPLE 1

A silver paste was spot-coated in the central portions of two main facesof disc-shaped β-alumina sintered body having a density of about 60%,supplied by Toshiba, (1.16+x/2)Na₂ O.11Al₂ O₃.xMgO; x≃0.8) having adiameter of 10 mm and a thickness of 3 mm, and the coated silver pastewas sintered to form silver electrodes. Simultaneously, silver lines areconnected to the silver electrodes, respectively. Then, a mixed pasteformed from 0.5 g of α-alumina (having an average particle size of 5 μm)and 2 ml of a saturated aqueous solution of Na₂ CO₃ (the Na₂ CO₃ /Al₂ O₃molar ratio was 1.2) was coated in a thickness of 1 mm on one main faceof the disc-shaped β-alumina in a region including the silver electrodeand having a diameter of about 8 mm, and the remaining surface of thedisc-shaped β-alumina was completely sealed and covered with aheat-resistant inorganic coating agent, "Ceramabond 503" (alumina typeinorganic paint supplied by Nissan Kagaku Co.), in a thickness of about1 mm. The entire assembly was heated and dried (at about 300° C.) by aninfrared lamp to obtain a gas sensor having a structure as shown in FIG.1.

This gas sensor was placed in a furnace maintained at 500° C. undervarious CO₂ partial pressures, and the CO₂ partial pressure (Pco₂)dependency of the generated electromotive force was examined. Theobtained results are shown in FIG. 2.

In FIG. 2, the difference ΔE mV between the electromotive forcegenerated in a standard gas having an N₂ /O₂ ratio of 4/1 and theelectromotive force generated in a gas to be measured, in which a partof N₂ is substituted by CO₂, is plotted on the ordinate and the CO₂partial pressure Pa is plotted on the abscissa.

As is apparent from FIG. 2, the CO₂ partial pressure dependency of theelectromotive force had a good linearity in a broad range and was inaccord with Nernst's equation. The apparent response speed was very goodand the response time was about 10 minutes either when the CO₂ partialpressure was increased or when the CO₂ partial pressure was decreased.Furthermore, these characteristics were stably maintained even after thelapse of 3 months.

Incidentally, even if the O₂ concentration in the standard gas wasreduced so that the O₂ partial pressure P_(O).sbsb.2 was 10⁻⁵ P_(a),linearity of the CO₂ partial pressure dependency (P_(CO).sbsb.2dependency) was not lowered.

If the reference electrode side of the disc-shaped β-alumina was opened,a gas response having reproducibility could not be obtained.

EXAMPLE 2

A gas sensor having a structure as shown in FIG. 1 was fabricated in thesame manner as described in Example 1 except that Li₂ CO₃ was usedinstead of Na₂ CO₃, and the CO₂ partial pressure dependency of thegenerated electromotive force was examined in the same manner asdescribed in Example 1.

The obtained results are shown in FIG. 3. In FIG. 3, white circlesindicate generated electromotive forces measured while the CO₂ partialpressure was being increased, and black circles indicate generatedelectromotive forces measured while the CO₂ partial pressure was beingdecreased.

As is apparent from FIG. 3 this gas sensor also showed a good linearityof the generated electromotive force in a broad range of the CO₂ partialpressure. The response speed was very good and the response time waswithin 5 minutes either when the CO₂ partial pressure was increased orwhen the CO₂ partial pressure was decreased. These excellentcharacteristics were maintained stably for a very long time.

CAPABILITY OR EXPLOITATION IN INDUSTRY

The gas sensor provided according to the present invention has a simplestructure, reduced size, and sufficiently stable operationcharacteristics. For example, this gas sensor can be used for detectionof air pollutants in a room, detection of the CO₂ concentration inexpiration, detection of the CO₂ concentration in the exhaust gas of anautomobile, and continuous measurement of the CO₂ concentration in thefermentation industry.

We claim:
 1. A gas sensor for measuring the concentration of CO₂, Cl₂,SO_(x) or NO_(x) in an ambient atmosphere comprising(a) a shaped solidelectrolyte; (b) a measurement electrode attached to a part of thesurface of said solid electrolyte; (c) a layer of a metal salt capableof forming a dissociative equilibrium with a gas component to bemeasured and which covers the exposed surfaces of both said measurementelectrode and said solid electrolyte adjacent said measurementelectrode; (d) a reference electrode attached to a part of the surfaceof said solid electrolyte other than that covered with said metal saltlayer; (e) a gas-intercepting layer covering the exposed surface of saidreference electrode and substantially all of the remaining surface, notcovered with said metal salt layer, of said solid electrolyte, said gasintercepting layer being substantially gas impermeable, and sealing saidreference electrode from the ambient atmosphere, and (f) lead lines fortaking out potentials from said measurement electrode and said referenceelectrode.
 2. A gas sensor as set forth in claim 1, wherein the solidelectrolyte is composed of Li/β-alumina, Li₁₄ Zn(CeO₄)₄, Li₅ AlO₄ ;Na/β"-alumina, Na/β-alumina, Na₂ Zr₂ PSi₂ O₁₂, Na₃ Zr₂ Si₂ PO₁₂,Na/β-Ga₂ O₃, Na/Fe₂ O; K/β-alumina, K₁.6 Al₀.8 Ti₇.2 O₁₆ ; or CaS.
 3. Agas sensor as set forth in claim 1, wherein the metal salt layer iscomposed of Li₂ CO₃, Na₂ CO₃, BaCO₃, SrCO₃, CaCO₃, MgCO₃, PbCO₃, FeCO₃,ZnCO₃, NaCl, AgCl, CaCl₂, PbCl₂, CdCl₂, CuCl₂, CoCl₂, MnCl₂, NiCl₂,FeCl₂, ZnCl₂, BiCl₂, MgCl₂, VCl₂ ; Na₂ CO₄, BaSO₄, SrSO₄, CaSO₄, Ag₂SO₄, CdSO₄, NoSO₄, ZnSO₄, CoSO₄, NaNO₃, Ba(NO₃)₂, Ca(NO₃)₂, or AgNO₃. 4.A gas sensor as set forth in claim 1, wherein the measurement electrodeor the reference electrode or both is composed of silver, gold,platinum, graphite, or a ferrite electrode material.
 5. A gas sensor asset forth in claim 1, wherein the gas intercepting layer is composed ofceramics, glass, or plastics.
 6. A gas sensor as set forth in claim 1,wherein the solid electrolyte is composed of β-alumina and the metalsalt layer is composed of an alkali metal carbonate.
 7. A gas sensor asset forth in claim 6, wherein the alkali metal carbonate is lithiumcarbonate, sodium carbonate, or potassium carbonate.
 8. A gas sensor asset forth in claim 6 or 7, wherein the metal salt layer is composed ofan alkali metal carbonate and β-alumina.
 9. A gas sensor as set forth inclaim 8, wherein the measurement electrode and the reference electrodeare composed of silver.